Chords are constructed using triads of the major scale.. So the G major scale notes are G. A. B. C. D. E. F#

So take the notes A. C. E. this makes an A minor chord,

My question is how do we know wich A. C. E note to play? There is a c note at the 1st fret B string another 3rd fret A string and so on and so on. I'm lost here